Eosinophilic pulmonary inflammation is a cardinal feature of asthma. Once in the lung, eosinophils (Eos) show an activated phenotype and become resistant to apoptosis after exposure to prosurvival cytokines such as GM-CSF. Long lived Eos contribute to tissue damage and facilitate the development of submucosal, bronchial fibrosis and airway remodeling. Therefore, suppression of GM-CSF production or its prosurvival signaling could reduce pulmonary eosinophilia and the long-term morbidity of asthma. GM-CSF released by activated Eos binds to a heterodimeric, cell surface receptor which activates multiple prosurvival cascades involving PI-3K, MAPK and Syk. How these pathways modulate Eos survival are largely unknown. Recently, we identified the peptidyl-prolyl isomerase (PPIase), Pin1, as a critical participant in GM-CSF mRNA metabolism as well as subsequent prosurvival signaling in Eos. Pin1 binds to and catalyzes the cis-trans isomerization of phosphoserine-proline or phosphothreonine-proline (Ser/Thr-Pro) peptide bonds. Isomerization of target proteins alters their conformation, function or stability. We now show that Pin1's isomerase activity is required for Eos survival in at least 2 ways. First, Pin1 controls the production of prosurvival cytokines by Eos by regulating the stability of GM-CSF mRNA. Pin1 binds to and directly controls the affinity of critical mRNA binding proteins for both GM-CSF mRNA and the RNA decay machinery in cells. Secondly, Pin1 is essential for the prosurvival effects of GM-CSF by directly interacting with multiple apoptotic effectors including Bax, PKC and procaspase 8. Therefore, we hypothesize that Pin1 is a critical, signaling intermediate which controls eosinophil survival through the regulation of GM-CSF production and GM-CSF prosurvival signaling. We therefore propose to: 1. Identify how Pin1 activity is regulated by GM-CSF mediated signaling. 2. Determine how Pin1 regulates the function of the GM-CSF mRNA binding protein, AUF1. 3. Characterize how Pin1 modulates the prosurvival effects of GM-CSF. 4. Determine if Pin1 blockade can reduce eosinophilic inflammation and preserve lung function in synergy with inhaled corticosteroids in animal models of asthma. In aggregate these studies will further our understanding of Eos biology in asthma. PUBLIC HEALTH RELEVANCE. Completion of the proposed studies will help understand how eosinophils die when growth factors are removed and potentially pave the way for the development of drugs which can accelerate that process. Such drugs could be useful for the treatment of chronic asthma.